The simplest and most straightforward way to determine how vestibular efferent neurons function in an intact animal is to record their activity during normal behavior and to record from the primary afferent neurons which they have been shown to influence. I propose to investigate the physiological characteristics and functional significance of efferent neurons in the vestibular system by recording from efferent and primary afferent neurons in intact, awake cats. Responses of these neurons to controlled vestibular (angular acceleration and head tilt) stimuli with the head immobilized will be compared with their responses to similar stimuli generated by free head movements which the animal has been trained to make. Comparison of the parameters of the transfer functions relating stimulus and response in the restrained and in free to move conditions will enable a test of hypotheses about efferent neuron functions. Do they for example regulate the discharge of afferent neurons in a way that increases their dynamic range and makes them more linear? Do the efferent neurons change the dynamic response properties of some primary afferents and not other? If so, how? Are the responses to rapid head movements and slow head movements equally effected? The vestibular labyrinth plays a central role in the reflexive control of eye movements, head movements, and posture; it contributes to the perception of space; it is undoubtedly involved in motion sickness, and in disease states may cause incapacitating nausea, dizziness, and nystagmus. The efferent neurons are an integral part of this sense organ, and their role in its function must be understood before understanding of the normal function of the vestibular system is complete. Although the anatomy of efferent projections of mammalian vestibular efferents has become better understood in the last 5 years with new histochemical techniques, the physiology of these neurons however, and the exact nature of their function in intact animals has remained enigmatic.